Metallization interconnects are critical to the proper electronic function of semiconductor devices. Several advances in semiconductor processing have been aimed at improving signal transport speed by reducing metal interconnect resistivities and improving resistance to electromigration effects. Copper has increasingly become a metal of choice in, for example, upper levels of metallization in a multi-level semiconductor device due to its low resistivity and higher resistance to electromigration. Tungsten (W), however, n is still preferred for use in the lower metallization layers adjacent to the silicon substrate since it provides an effective diffusion barrier to metal diffusion from overlying metallization layers to react with the silicon substrate. Tungsten further has high resistance to electromigration and can effectively be used to fill high aspect ratio vias by chemical vapor deposition (CVD) processes.
According to prior art processes, an oxide layer, referred to as a first layer oxide or an inter-layer dielectric (ILD) is deposited following forming of CMOS transistors with protruding polysilicon gate electrodes. The first ILD layer fills the gaps between the polysilicon electrodes, followed by a CMP process to planarize the layer prior to forming tungsten plugs. Problems with prior art processes include the practice of using a doped oxide, for example doped with phosphorous or boron to provide a binary glass which improved the dielectric properties and which be heated and flowed following deposition to improved the topography planarity of the surface. Problems with using binary glasses is that they are frequently much softer than SiO2 and have a high removal rate in a CMP process thereby making control of a final thickness of the first ILD layer including tungsten plugs difficult to control.
Therefore, there is a need in the semiconductor processing art to develop an improved ILD layer stack to maintain the advantages of doped oxide binary glasses while improving a structural stability including improved CMP thickness control.
It is therefore an object of the invention to provide an improved ILD layer stack to maintain the advantages of doped oxide binary glasses while improving a structural stability including improved CMP thickness control, while overcoming other shortcomings of the prior art.